KR20080080609A - Etherified melamine-formaldehyde condensates with a high solids content and low viscosity - Google Patents

Abstract

A process for preparing etherified melamine-formaldehyde condensates, in which a methylolation of melamine is first performed with a formaldehyde component in a molar ratio of 1:6 to 1:15, preferably at a pH of > 7. This is followed by an etherification of the resulting hydroxy-methylation intermediate in the presence of a C1-C20-alcohol, preferably of a C1-C6-alcohol, more preferably of a C1-C4-alcohol, at a pH of < 4.5, and subsequently by at least one distillation step in order to remove methanol in particular from the system, wherein a pH of > 9.5 is established before, during and/or after the distillation. According to the invention, this is followed by several hydroxymethylation, etherification and distillation steps, in order to achieve an etherified melamine-formaldehyde condensate which features a high degree of hydroxymethylation, a high solids content and a low viscosity.

Description

Etherified melamine-formaldehyde condensate with high solids content and low viscosity {ETHERIFIED MELAMINE-FORMALDEHYDE CONDENSATES WITH A HIGH SOLIDS CONTENT AND LOW VISCOSITY}

The present invention relates to a process for the preparation of etherified melamine / formaldehyde condensates, etherified melamine / formaldehyde condensates and their use. Specifically, the present invention relates to a method suitable for effecting the ratio of the mononuclear component of substitution degree <6 in the etherified melamine / formaldehyde condensate, thereby affecting structure, reactivity and viscosity. Here, the mononuclear component refers to the melamine molecular building block, and the degree of substitution refers to the number of hydrogen atoms in the amine group of the melamine molecular building block (melamine ring) exchanged by reaction with formaldehyde.

Etherified melamine / formaldehyde condensates are known and are described, for example, in Houben-Weyl, Methoden der organischen Chemie, Volume XIV-2, 1963, pages 319-402 and Ullmann's Encyclopaedie der Technischen Chemie, 1953, Volume 3, pages 487-489, and also melamine resins, melamine resin condensates or melamine / formaldehyde resin condensates are used as synonyms.

Industrial demand for etherified melamine / formaldehyde condensates has increased significantly. The etherified melamine / formaldehyde condensates are aminoplast crosslinkers in coating combinations, dispersing binders containing hydroxyl groups and crosslinkers for latexes for the production of compression mats, and for surface shaping of sheet materials. And as a component of waterproof adhesives for bonding veneers in the manufacture of laminates, edge bands. In addition, etherified melamine / formaldehyde condensates are also used in the paper industry for impregnation and coating of paper.

For various applications in the aforementioned areas, surface hardness, scratch resistance, dry thermal stability, water vapor resistance to melamine resins formed and cured in a curing process performed by the action of acid and / or heat. Adhesive strength and resilience are desired.

The preparation of etherified melamine / formaldehyde condensates is disclosed, for example, in DE-A 25 16 349 and US Pat. No. 4,425,466, wherein the melamine is formaldehyde and alcohol in the presence of an organic strong acid at 80 ° C to 130 ° C. React with

In addition, the prior art discloses a number of manufacturing methods for melamine / formaldehyde resin condensates, which methods were based on different requirements. Thus, DE-1 102 61 804 has a mean molar mass of 500 to 50,000, melamine without hydroxy methylene amino group bonded to the triazine ring and -NH-CH ₂ -O-CH ₂ -NH- group linked to the triazine ring. Direct synthesis methods for the preparation of resin condensates are described.

Other publications, for example DE-A 100 07 951 and EP-A 0 385 225, relate to the storage stability of melamine / formaldehyde condensates or their suitability to the coating of wood-based materials and their conductivity.

DE-A 32 16 927 comprises at least 80% by weight of melamine and formaldehyde containing 1 to 20% by weight of alkali metal disulfite based on the solids content of the melamine resin and having a molar ratio of 1: 1.5 to 1: 4. Describe the solution of melamine resin. Here, Example 1 recommends that the melamine resin be an aqueous solution by monitoring the viscosity appearing during condensation of melamine and formaldehyde. Melamine resins thus prepared are recommended for the production of filaments and fibers and have a viscosity of greater than 10,000 mPa · s.

It is an object of the present invention to provide a process for the preparation of etherified melamine / formaldehyde condensates, in particular marked by high solids content and low viscosity.

The purpose is

(a) as a first reaction step, melamine is methylolated with a formaldehyde component in a molar ratio of 1: 6 to 1:15;

(b) as a second reaction step, the hydroxymethylated intermediate produced in step (a) is converted to a C ₁ -C ₂₀ alcohol, preferably a C ₁ -C ₆ alcohol, particularly preferably a C ₁ -C ₄ alcohol. Etherification in the presence at pH <4.5;

(c) as a third reaction step, adding an inorganic base to the reaction mixture of step (b) to adjust the pH to> 9.5 and removing volatile components, in particular methanol, by one or more distillation steps;

(d) repeating steps (b) and (c) one or more times;

It is achieved by a process for the preparation of etherified melamine / formaldehyde condensates.

By using excess formaldehyde in the process according to the invention for the preparation of melamine / formaldehyde condensates, hydroxymethylation can be present at all reaction stages. Melamine / formaldehyde condensates prepared according to the present invention have high degree of hydroxymethylation and degree of etherification. There is also a relationship between etherification and viscosity. Melamine / formaldehyde condensates having a mononuclear component having a degree of substitution close to 6 in a small proportion have an unsubstituted amino group in a small proportion and exhibit beneficial viscosity.

In the process according to the invention, melamine is preferably used as a solid. For methylolation, a formaldehyde component, for example formaldehyde, or a mixture of formaldehyde, water and methanol, for example in a proportion of 50% by weight formaldehyde, 0-10% by weight water and 40-50% by weight methanol Use Preferably, formaldehyde is used in the form of an aqueous solution or a form of paraformaldehyde at a concentration of 30 to 85% by weight, preferably 40 to 60% by weight. Here, 6 to 15 moles, preferably 7 to 10 moles of formaldehyde are used per mole of melamine.

The methylolation of melamine is advantageously carried out at a temperature of 40 to 100 ° C, preferably 60 to 85 ° C. Methylolation of melamine can begin when the melamine is mixed with the formaldehyde component without having to initially change the pH in the acidic range. Preferably, methylol is carried out at pH> 7, preferably at pH 8 to 10. The pH can be set using inorganic bases such as sodium hydroxide solution or potassium hydroxide solution, or amines such as monoethanolamine or triethyleneamine. If appropriate, the batch is buffered with, for example, Borax. Methylolation preferably takes place at atmospheric pressure and it is also possible to carry out under pressures of typically 0.5 to 10 bar.

In the following second reaction step, ie the first esterification step, the hydroxymethylated intermediate obtained from the methylolation step is selected from C ₁ -C ₂₀ alcohols, preferably C ₁ -C ₆ alcohols, particularly preferably C _1- Etherize at pH <4.5 in the presence of C ₄ alcohol. Preferably methanol is used as the alcohol for the etherification. Here, for example, hydroxymethylated intermediates can be added to acid methanol, ie a methanol / water mixture with an acidic pH, preferably 1: 7 to 1:50 per etherification step, preferably 1: Methanol excess is used with melamine: methanol ratio of 7 to 1:25. Alternatively, hydroxymethylated intermediates can be added to methanol and the pH is preferably pH <7, with only the addition of a suitable acid such as HNO ₃ , H ₂ SO ₄ , HCl or methanesulfonic acid. Preferably in the range of pH 2.5 to 3.5.

The etherification takes place preferably at a temperature of 40 to 120 ° C., preferably 50 to 80 ° C., at a prevailing pressure of 0.5 to 10 bar, preferably 1 to 3 bar. The reaction time can vary from 10 to 120 minutes, preferably 20 to 60 minutes.

Alternatively, the methylolation and etherification steps can be combined in one reaction step, with a temperature in the range of 40 to 150 ° C., preferably 80 to 120 ° C., and a pressure of 1 to 10 bar, preferably Is set to 1 to 3 bar and the pH is lowered during the reaction to <7.

Thereafter, further hydroxymethylation is brought about by adding an appropriate inorganic base such as aqueous NaOH or KOH solution to the reaction mixture present after the first etherification step to impart alkalinity. The pH is set to> 9.5, preferably in the range of 10 to 11.

The reaction mixture, preferably methanol-based melamine-formaldehyde condensate solution, present after etherification is applied to at least one distillation step. Here, volatile components, in particular excess methanol, are removed. The distillation step can preferably be carried out by vacuum distillation, with a pressure of 1,000 to 0.1 mbar, preferably 1,000 to 10 mbar and a temperature of 30 to 70 ° C prevail.

An additional distillation step can be followed, which makes it possible to remove water and excess formaldehyde part.

In order to achieve the complete conversion of melamine to hexahydroxymethylmelamine as completely as possible, in the prior art, an alkaline pH is appropriately set and 0.01-3 mol, preferably 0.01-1 mol of formaldehyde per 10 mol of melamine is 10 to Additional methyl by adding at a temperature of 30 to 100 ° C., preferably 30 to 70 ° C. and a pressure of 0.5 to 10 bar, preferably 1 to 3 bar over a time of 120 minutes, preferably 10 to 60 minutes It is disclosed to carry out the oligration and etherification steps. The same or different alcohols can be used here for further etherification steps, resulting in mixed etherification products. This may be followed by a concentration step using a distillation step. However, the system has been found to be more difficult to handle when increasing the degree of hydroxymethylation of melamine. In particular, the processing procedure in the vessel becomes very complicated until the viscosity of the reaction mixture rises and can no longer be carried out. The proportion of hexahydroxymethylmelamine present as a solid should be as high as possible according to the invention, which is only difficult and thoroughly mixed by the stirrer.

The reaction is preferably carried out in a stirring vessel and sufficient mixing in the system is achievable by suitable choice of the stirrer and the input energy used.

In an embodiment according to the invention, the further hydroxymethylation step takes place by the excess formaldehyde present, preferably in the first etherification step.

Here, it was found that methanol present in the hydroxymethylated intermediate significantly reduces the reactivity of formaldehyde due to the formation of hemiacetal. Thus, if excess methanol is removed from the solution in advance by a suitable distillation step, hydroxymethylation is particularly effective in terms of high degree of substitution. In addition, hydroxymethylation occurs especially in the alkaline pH range. However, it has been found that the pH decreases during the distillation step for the removal of methanol, possibly as a result of the Cannizzaro reaction, ie, the disproportionation of formaldehyde into methanol and formic acid. In addition to the effect on the pH of the solution, the available formaldehyde ratio is likewise reduced.

Thus, for the reaction according to the invention, it is important to remove the methanol by a distillation step and then to bring the pH of the reaction mixture to> 10. This can be achieved by adding an amount of base so that the corresponding pH can be maintained before methanol distillation. Alternatively, the base can be metered in during and / or after methanol distillation.

It has also been found that the method of distilling more quickly is particularly suitable for maintaining the pH in the desired alkaline range during methanol distillation.

Distillation is particularly advantageously carried out in falling-film evaporators, thin-film evaporators and SAMBAY thin film evaporators. The last type of evaporation, in which the liquid layer or liquid membrane is provided mechanically, is particularly suitable for the distillation of temperature sensitive solutions and typically high viscosity high boiling solutions. The liquid to be evaporated is distributed as a thin layer, preferably over a cylindrical heat exchange surface, by means of a rigid stirring blade or movable wiper of the rotor, so that an increase in the viscosity of the solution to be evaporated, Does not give a problem.

In addition, metering large amounts of base prior to methanol distillation demonstrates that it is beneficial to maintain the pH at> 10, preferably> 11 after the distillation step.

If necessary, a second metering of the base may be carried out after methanol distillation.

In the process according to the invention, there may be a further reaction step for methylolation and etherification, followed by a first methylolation and etherification step followed by methanol distillation, wherein the reaction mixture, methanol-based melamine / formaldehyde The condensate solution is prepared in an acidic pH range, preferably in the range of pH 2.5 to 3.5, a reaction temperature of 40 to 120 ° C., preferably 40 to 70 ° C., and a duration of 10 to 60 minutes, preferably 10 to 30 minutes. Condensation with methanol. Alkaline is imparted to the system before, during or after the subsequent methanol distillation. Distillation is carried out at a temperature of 30 to 120 ° C., preferably 40 to 70 ° C., and a pressure of 1,000 to 0.01 mbar, preferably 1,000 to 10 mbar.

The multi-step method has proved to be beneficial in achieving the hydroxymethylation degree of the etherified melamine / formaldehyde condensate as high as possible. In particular, removing excess methanol in the system and maintaining an alkaline pH increases the effectiveness of each hydroxymethylation step. This multi-step procedure avoids handling problems with the reaction mixture due to the increase in solids content with gradual hydroxymethylation.

According to the invention, the use of an additional hydroxymethylation step, in each case, with subsequent methanol distillation and target control of pH before, during and after methanol distillation, leads to a high in the etherified melamine / formaldehyde condensate. The degree of hydroxymethylation can be achieved.

In addition, according to the invention, the effect that the degree of substitution depends on the viscosity of the final product to be achieved is also used. As the degree of hydroxymethylation increases, the viscosity of the final product decreases, as a result of which the subsequent processing steps are simplified and the potential use of the etherified melamine / formaldehyde condensation according to the invention, e.g. Potential uses as acid curable crosslinkers for binder systems including hydroxyl groups for clearcoats, colored coatings, wood coatings, fortification or coil coatings are expanding.

The present invention will now be described in more detail with reference to examples. Unless explicitly stated otherwise, all data for parts and percentages are weight data. After each etherification step, the degree of substitution and degree of etherification were analyzed based on the selected peak and HPLC, and the solids content and viscosity were measured.

The names in the X / Y / Z derivatives are as follows:

X corresponds to the mole of melamine,

Y corresponds to the mole of formaldehyde,

Z corresponds to moles of methanol.

For example, data 1/7/6 means that 7 mol of formaldehyde and 6 mol of methanol per 1 mol of melamine are incorporated. This indicates that additional formaldehyde attacks existing hydroxymethyl groups, resulting in the formation of small polyoxymethyl chains.

Example  One

Initially 375 g of 40% aqueous formaldehyde solution was placed in a reaction vessel, adjusted to pH 9 with NaOH to form a 25% aqueous solution. After addition of 70 g of melamine, the reaction mixture was heated to 70 ° C. and stirred from 50 ° C. for 40 minutes. The obtained hydroxymethylated intermediate was then diluted with 167 g of methanol, adjusted to pH 3 with 3.2 ml of 30% strength HNO ₃ aqueous solution and etherified at 60 ° C. for 30 minutes. The etherification reaction was terminated by addition of 2.8 ml of 25% sodium hydroxide solution to adjust the pH to> 10. Excess methanol was distilled off in vacuo at a pressure of 80 mbar and a temperature of 90 ° C.

The solid content measured by drying in a drying oven at 120 ° C. for 2 hours was 78.1%, the water content was 3%, and the viscosity was measured as 75,000 mPa · s.

HPLC analysis revealed the following derivatives, the remainder being of dimer and high molecular weight, making it impossible to designate them as individual species.

1/4/4 1/5/4 1/6/4 1/5/5 1/6/5 1/6/6 1/7/6 2 / X / X (Mol / mol / mol) 1.7 7.4 11.9 3.9 11.5 6.4 1.8 7.1 (area%)

The substitution was still not very complete and only 50% of the product was identified and the rest could not be specified.

Then, the obtained etherified melamine / formaldehyde condensate was diluted with 267 g of methanol, the pH was adjusted to 3 with 1.8 ml of 30% HNO ₃ aqueous solution, and condensation was carried out at a temperature of 60 ° C. for 12 minutes. It was. Thereafter, 1.5 ml of a 25% aqueous NaOH aqueous solution was added to set the pH to 10.5 in the reaction mixture, and excess methanol was distilled off under vacuum at a pressure of 120 mbar and a temperature of 90 ° C. The solids content measured in the drying oven (120 ° C., t = 2 h) was now 95.4%, the water content was 0.8% and the viscosity was 15,500 mPa · s.

HPLC analysis showed the following results:

1/4/4 1/5/4 1/6/4 1/5/5 1/6/5 1/6/6 1/7/6 2 / X / X (Mol / mol / mol) 0.3 1.3 1.9 5 13.3 18.1 5.2 46.5 (area%)

The product was then diluted again with 267 g of methanol at pH 3 set by 1.2 ml of 30% strength HNO ₃ aqueous solution. Condensation was carried out at a temperature of 60 ° C. for 12 minutes. Thereafter, the solution was given alkalinity with 0.9 ml of a 25% NaOH aqueous solution, and excess methanol was distilled off under vacuum at a pressure of 120 mbar and a temperature of 90 ° C.

The solids content, measured by drying in a drying oven at 120 ° C. for 2 hours, is now 97.8%, the water content is 0.5%, and the viscosity is measured at 6,400 mPa · s.

HPLC analysis showed the following results:

1/4/4 1/5/4 1/6/4 1/5/5 1/6/5 1/6/6 1/7/6 2 / X / X (Mol / mol / mol) 0.4 1.7 0.9 0.6 9 31.9 8 28.2 (area%)

This example shows that the proportion of fully etherified product increases step by step (1/4/4, 1/5/5, 1/6/6) and at the same time the viscosity of the reaction mixture is 75,000 mPa.s It clearly shows that it decreases to 6,400 mPa.s.

Claims (9)

(a) as a first reaction step, melamine is methylolated with a formaldehyde component in a molar ratio of 1: 6 to 1:15; (b) as a second reaction step, the hydroxymethylated intermediate produced in step (a) is etherified at pH <4.5 in the presence of C ₁ -C ₄ alcohol; (c) as a third reaction step, adding an inorganic base to the reaction mixture of step (b) to adjust the pH to> 9.5 and removing volatile components, in particular methanol, by one or more distillation steps; (d) repeating steps (b) and (c) one or more times; Process for the preparation of etherified melamine / formaldehyde condensates. The process of claim 1 wherein the methylolation (a) is carried out at pH> 7. The process for the preparation of etherified melamine / formaldehyde condensates according to claim 1 or 2, wherein methanol is used as the alcohol in etherification (b). The etherified melamine / formaldehyde condensation according to any one of claims 1 to 3, wherein the inorganic base is metered into the reaction mixture after the distillation step of reaction step (c) and set to pH <9.5. Method of making water. The etherified process according to any one of claims 1 to 4, wherein the one or more distillation steps of reaction step (c) are carried out in a falling-film, thin-film or SAMBAY evaporator. Method of Making Melamine / Formaldehyde Condensates. The etherified melamine prepared by the method according to any one of claims 1 to 5, wherein the proportion of mononuclear components having a degree of substitution <6 is less than 10% based on the total amount of the melamine / formaldehyde condensate. Formaldehyde condensates. An etherified melamine / formaldehyde condensate, which may be prepared by the process according to any one of claims 1 to 5, wherein the solids content is greater than 96%. An etherified melamine / formaldehyde condensate, which may be prepared by the process according to any one of claims 1 to 5, having a viscosity of less than 3,000 mPa · s. Use of an etherified melamine / formaldehyde condensate prepared according to any one of claims 1 to 5 as an acid curable crosslinking agent.